1. Field of the Invention
This invention relates to an apparatus and method for fusion joining a pipe with respect to a fitting, and more particularly to an improved apparatus and method wherein a novel heating element is insertably positioned between a fitting and a portion of the pipe.
2. Description of Prior Art
Various conventional apparatus and methods exist for fusion joining polyethylene pipes, such as those that employ heat fusion, electrofusion or induction fusion techniques. Electrofusion uses a conventional resistive heating method wherein a wire spiral coil is typically internally moulded within the thermoplastic fitting. An electrical current is applied directly to the wire coil via terminals on the fitting which results in the coil heating the thermoplastic material to a temperature which softens the thermoplastic material enough to fuse the pipe to the fitting.
In GB1149716 there is described an electrical resistance heating coil for use as a welding sleeve for joining plastics pipes to fittings wherein the pipe and fitting describe an annulus within which the welding sleeve is mounted. The coil comprises an electrical resistance wire embedded in a sheath of plastics material of precisely defined dimensions in order, it is said, to provide adequate welding efficiency. The proposed design complicates manufacture and it is difficult to connectorise the coil to an external power source. In addition, the existing wires provide potential leak paths from the joint region.
In a conventional induction fusion method for joining a pipe and a fitting, a heating element is moulded within the thermoplastic fitting. Current flow is induced through the heating element by a primary induction coil that is positioned around the portion of the fitting that contains the heating element. The temperature of the heating element is raised enough to soften or melt the fitting material and the pipe material, thereby resulting in fusion between the fitting and the pipe. Examples of induction fusion methods and apparatus are described in DE-A-2136656, EP-A-026191 and EP-A-513391. Such apparatus require a fitting with a moulded-in heating element. Thus, such methods cannot be used with existing conventional plastics fittings that do not contain a moulded-in heating element.
GB 2272663 describes a plastics pipe having an enlarged diameter portion at one end, which is provided at its inner surface or in the vicinity of its inner surface with a heating unit. A second plastics pipe to be joined is inserted into the enlarged portion and the heating unit energised. In manufacturing the pipe having the enlarged diameter portion, an end of the pipe is heated and softened, the enlarged socket formed, and the heating generating unit incorporated, in one embodiment, by pressure-inserting the heat generating unit on a core mould into the socket. The heat generating unit can be an electrofusion coil embedded in a plastics material, or a bare induction fusion element.
The electrofusion heat generating unit of GB2272663 is inconvenient, due to the necessity of providing a large flange to accommodate the terminals of the coil, difficult to overmould and expensive to manufacture. The bare induction fusion element, even when pressure inserted into the socket, is not completely surrounded by plastics material and is liable to overheat and be subject to thermal runaway. Once thermal runaway has started in one part of the element, its resistance changes dramatically and that area of the element draws in current (voltage, power) such that the rest of the element will not heat. The overheated region of the element can also cause local degradation of the thermoplastic material. In such cases usually the melt region cannot be contained, and thus a good fusion joint cannot be made.
The entire disclosures of all the above mentioned patents are incorporated herein by reference for all purposes.
It is apparent that there is a need for an apparatus and method that can be used to fuse a fitting with respect to a pipe, using induction techniques and a conventional fitting that does not contain a moulded-in heating element.